I tried to paint about a 12 inch line next to the walls. I did this with a brush and for lack of better description just kinda slapped/slopped it on. My epoxy flowed pretty well and in seconds you couldn't see any brush strokes. I did this all the way around the tapped edge of the foundation that I was working so as to not have to come back to the brush. I would then start in a corner and work towards the middle. I tried to paint a swath about 10 ft. wide by 4 ft deep. That was about all the further I could throw decorative flakes with any type of uniformity. I tried to overlap the ending and the beginning (the 4 ft. area) by 10 inches or so. I didn't end up with any thin spots so it must have worked O.K. I would cover the roller pretty thoroughly by dipping in a 5 gallon bucket and apply light pressure, just enough to make sure the nap was getting into the concrete. I found that lots of paint on the roller worked well for me.
I'm sure that the pot life varies from product to product but I found that mine would start to get pretty thick after 30 or 40 minutes. I ended up mixing smaller amounts but adding new mix to the same bucket more often. This would thin the thickening mix and made it so that I had fluidity throughout the process.
I think that the line problem would be dependent upon how well your chosen product flows. The stuff I used would just blend and it was never an issue. If you're having a problem with lines maybe the mix is a little thick?